Maybe I'm all wet but I think phorwath is questioning the same thing. I'm asking is there enough steel all around that big fat cartridge using a 700 action to be safe? I'm pretty sure several gunsmiths that post here refuse to build around any cartridge fatter than the RUM size. It's the bolt, lugs, receiver, and barrel shank diameter in that action that is in question.

Enough steel where ? don't know what you mean ! give me a valid question so I can answer it !

many 338 Lapuas have been built on the Rem700 actions and including 7.82 Warbirds, both operate at higher pressures than the 500 Jeffery

Rem 700's can be easily made to run the parent 500 Jeffery cartridge, don't see why necking it down to 375 cal. would make a difference !
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The larger the inside diameter of the cartridge case, the greater the force that will be applied to the bolt face, and to the bolt lugs against the receiver. Force = Pressure X Area. Increase the cross-sectional area and the force will increase proportionally for any given pressure. The 0.621" outer diameter of the 500 Jeffery case, versus the 0.588" outer diameter for the .338 Lapua case, will generate approximately 12.5% greater force on the bolt lugs, given equal case pressures.

Expressed another way, to keep bolt lug force similar to the 338 Lapua case, you'll have to reduce 500 Jeffery case pressure 12.5% less than 338 Lapua case pressure.

There are gunsmiths that have posted on this Forum that refuse to chamber the smaller diameter 338 Lapua cartridge in a Remington 700 action because they've experienced steel yielding (deforming) where the bolt lugs contact the receiver with the 700 action. This yielding of the steel results in growing/increasing cartridge headspace. Doesn't mean you can't find someone that might be willing to build the 500 Jeffery-based wildcat on the Rem 700. It does mean that some will refuse to take on the risk of liability for chambering the 500 Jeffery case in a 700 action.

You may be asked to consider using a larger framed bolt action. A Weatherby Mark 5 action would probably be a better fit for this large diameter casing. I'm sure there are additional options, including expensive custom actions.

From Wikipedia:
"There have been a few rifles chambered in the 500 Jeffery including Jeffery, Heym, CZ-USA, and a few single shots including Ruger No.1, and the Butch Searcy & Co.. Mauser offers its Model 98 magnum in the caliber. Its "Elephant" model is offered in 500 Jeffery exclusively. In 2011, Sako began offering the caliber in its Model 85 "Safari" rifle using a new XL size action."

No mention of any commercial rifle manufacturer chambering the 500 Jeffery in a Rem 700-sized bolt action.

The Rem 700 was a consideration simply because I have a blued 300 RUM available to chop up,
it is definitely a point in discussion as I had the same questions on the integrity of it, Remington builds guns in 338 Lapua on their 700 and the big Jeff works on lower pressures than the hopped up 338 Lapua to begin with,

also the boltface will be less alteration from .534" to .575" versus the Lapua at .589"... I was told by several smiths it would work just fine so long as the pressures are maintained for the original cartridge.

My other (and original) option is to use a Sako M995 which already has the proper boltface.
I have two Sakos one in 7.82 Warbird and another in 30-378 WBY so either one will work just less aftermarket options which led me to the M700..

I welcome all input and especially concerns regarding safety..

__________________
How is it y'all still don't know the difference between CALIBER and CARTRIDGE ??

"In guns, particularly firearms, caliber or calibre is the approximate internal diameter of the barrel, or the diameter of the projectile it fires."
WIKIPEDIA

"Fear of weapons is a sign of retarded sexual and emotional maturity"
Sigmund Freud